Analog signal processing is commonly performed in many applications. One example is a prosthetic hearing device which processes ambient sound to supplement or provide hearing ability to individuals with various types of hearing impairments.
In recent years, rehabilitation of sensorineural hearing disorders with implantable electronic systems has acquired major importance. In particular, this applies to the group of patients in which hearing has completely failed due to accident, illness or other effects, or in which hearing is congenitally non-functional. If, in these cases, only the inner ear (cochlea), and not the neural auditory path which leads to the brain, is affected, the functional auditory nerve may be directly stimulated with electrical stimulation signals to provide a hearing perception, which may lead to sound or speech comprehension.
In these so-called Cochlear™ implants, an array of stimulation electrodes is inserted into the recipient's cochlea. This array is controlled by an electronic system encased in a hermetically sealed, biocompatible housing implanted in the mastoid. The electronic system essentially contains decoder and driver circuits for the stimulation electrodes. Acoustic sound reception and conversion of acoustic signals into electrical signals typically takes place externally in a speech processor worn by the recipient. The speech processor superimposes the preprocessed signals, properly coded, on a high frequency carrier signal which, via inductive coupling, is transmitted transcutaneously to the implant through the closed skin. A microphone is located outside of the body, typically in a behind-the-ear housing worn on the external ear. The microphone is typically connected to the speech processor by a cable.
With prosthetic hearing devices as well as other similar or related audio signal processing applications, there has been a trend to process digital representations of an analog signal rather than the analog signal itself. This process, commonly referred to as digital signal processing (DSP), involves the conversion of analog signals to digital signals. A/D conversion is often performed along with other analog signal conditioning or amplification operations. As such, these initial operations are sometimes referred to as front-end operations, and the hardware and/or software components that perform such initial operations are sometimes collectively referred to as a “front end” of the speech processing pipeline.